<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>知了社区</title>

</head>
<body>
<!--头部导航栏-->
   <div id="header">

   </div>
    <style>
    #header{
        width: 100%;
        height: 40px;
         background-color: rgba(0,0,0,0.5)
    }
    .logo_img(display:inline-block;)}
        #logo h1{
            display: inline-block;
        }
     </style>
</body>
</html>